DE102020209052A1 - CORROSION-RESISTANT OXIDE FILMS AND APPLICATION FOR BIPOLAR FUEL CELL PLATE - Google Patents

Abstract

Corrosion resistant oxide films for use with proton exchange membrane fuel cells are described. Proton exchange membrane fuel cell bipolar plates are exposed to highly acidic environments which can affect the bulk material and related properties of the bipolar plate, resulting in reduced proton exchange membrane fuel cell lifetimes. Materials, structures, and techniques for increasing the corrosion resistance of bipolar plates are disclosed. Such materials include substrates having a surface area that includes an Fe2O3 oxide layer with (110), (012), or (100) Fe2O3 surface facets configured to impart corrosion resistance properties to the substrate.

Description

TECHNICAL AREA

The present disclosure is directed generally to corrosion resistant oxide films for use in proton exchange membrane fuel cells. In particular, the present disclosure relates to proton exchange membrane fuel cell bipolar plates having corrosion resistant substrate surface structures made of iron oxide and to methods of making the same.

BACKGROUND

Fuel cells, and in particular proton exchange membrane fuel cells (PEMFCs), show promise as energy sources with high efficiency, high power density, relatively light weight and zero carbon dioxide emissions for use in a wide range of applications. Such applications include, but are not limited to, transportation, stationary power generation, and portable power generation. Particularly relevant for its applications in automobiles and for applications associated with other modes of transport, the PEMFC represents an environmentally friendly alternative to internal combustion engines for a large number of vehicles.

PEMFCs work on the basis of the transfer of protons between an anode and a cathode. Key components of PEMFCs include a proton exchange membrane, through which protons are transferred, and a membrane-electrode assembly (MEA), in which the proton exchange membrane is enclosed. PEMFCs also include bipolar plates (BPPs) that connect and disconnect individual fuel cells in series to form a fuel cell stack. Among other functions, BPPs provide the necessary voltage, help distribute fuel or propellant gas and oxygen over the active surface of the MEA, and conduct electrical current from the anode of one cell to the cathode of the next within the stack. With regard to such functionality, BPPs must not only be sufficiently chemically inert to withstand degradation in the highly corrosive environment of the fuel cell, but also sufficiently electrically conductive to allow electron transfer for the oxygen reduction reaction of the fuel cell.

BPPs can make up 60 to 80% of the weight of the PEMFC stack and are among the most expensive PEMFC components, often accounting for between 25% and 45% of the stack cost. Although other metals such as titanium and aluminum can be used, BPPs are typically made from stainless steel. Since PEMFC operation typically takes place in highly acidic environments, BPP materials with high corrosion resistance are desirable for long term PEMFC operation. Treatment techniques such as the introduction of conductive oxide and / or nitride coatings for stainless steel BPPs can help improve their lifespan in the acidic PEMFC environment. Despite the potential of such techniques, the corrosive formation of Fe ₂ O _{3 is} inevitable when Fe metal is exposed to water and oxygen, as occurs under the acidic operating conditions of the PEMFC environment. Accordingly, materials, structures, and techniques for increasing the corrosion resistance of BPP materials are desired.

SHORT VERSION

In at least one embodiment, a corrosion resistant substrate is disclosed. The substrate may have a bulk region and a surface region comprising an Fe ₂ O ₃ oxide layer with (110), (012), or (100) Fe ₂ O ₃ surface facets configured to impart corrosion resistance properties to the substrate , lock in. In some embodiments, the Fe ₂ O ₃ oxide layer is between 0.001 and 0.5 µm thick. Furthermore, the Fe ₂ O ₃ oxide layer of the corrosion-resistant substrate can be formed by a surface morphology with a first surface facet group that includes (110), (012) or (100) -Fe ₂ O ₃ surface facets, and a second surface facet group that includes (001) - or (101) -Fe ₂ O ₃ surface facets, be characterized in such a way that the Fe ₂ O ₃ oxide layer is predominantly built up from the first group of surface facets. In accordance with one or more embodiments, the surface area of the corrosion resistant substrate can include a protective coating comprising MgO, Al ₂ O ₃ , TiO _2, or ZrO ₂ . In another embodiment, the surface area of the corrosion resistant substrate can include a protective coating formed from ternary (or higher) chemical compounds such as, for example, ABO _x , where A is Mg, Al, Ti or Zr, B is Zn, Sn , Cr or Mo and x is an integer ranging from 1 to 8.

In another embodiment, a bipolar plate (BPP) for a proton exchange membrane fuel cell (PEMFC) is disclosed. The BPP may be a corrosion-resistant substrate that includes a bulk area and a surface area comprising an Fe ₂ O ₃ oxide layer with (110), (012), or (100) -Fe ₂ O ₃ surface facets configured to imparting corrosion resistance properties to the substrate. In some embodiments, the Fe ₂ O ₃ oxide layer is between 0.001 and 0.5 µm thick. The Fe ₂ O ₃ oxide layer of the corrosion-resistant substrate can be formed by a surface morphology with a first surface facet group including (110), (012) or (100) -Fe ₂ O ₃ surface facets and a second surface facet group including (001 ) or (101) -Fe ₂ O ₃ surface facets, be characterized in such a way that the Fe ₂ O ₃ oxide layer is predominantly built up from the first group of surface facets. The BPP substrate can be constructed from stainless steel. In one or more embodiment (s), the surface area of the BPP substrate can include a protective coating comprising MgO, Al ₂ O ₃ , TiO _2, or ZrO ₂ . In another embodiment, the surface area of the BPP substrate can include a protective coating formed from ternary (or higher) chemical compounds such as ABO _x , where A is Mg, Al, Ti or Zr, B is Zn, Sn , Cr or Mo and x is an integer ranging from 1 to 8. In one or more of the embodiments, the corrosion resistance of the surface area of the BPP substrate is less than 1 µA cm ^-2 at 80 ° C, a pH between 2 and 3, and in the presence of about 0.1 ppm HF.

In yet other embodiments, methods of making corrosion resistant substrates are disclosed. In at least one embodiment, the method may include cleaning a stainless steel substrate with an organic solvent and electrochemically oxidizing the stainless steel substrate to form a corrosion resistant surface area having an Fe ₂ O ₃ oxide layer with (110) -, (012 ) - or (100) -Fe ₂ O ₃ surface facets. In another embodiment, the method may include growing the Fe ₂ O ₃ oxide film on a stainless steel substrate by using a solution based method. In one example, hydrolysis at 80 to 100 ° C in a water bath with a different aging time of 1, 2, 3, 4, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55 or 60 minutes. In other examples, hydrolysis is carried out at 25 to 100 ° C. in a water bath with an aging time between 1 and 120 minutes.

Figure list

• The 1 FIG. 10 is a schematic representation of a proton exchange membrane fuel cell including a bipolar plate in accordance with one or more embodiments;
• The 2 Figure 12 is a perspective view of a non-limiting example of a bipolar plate having a surface area that includes a corrosion resistant iron oxide structure in accordance with one or more embodiments;
• The 3A to 3E represent morphologically significant atomic structures of α-Fe ₂ O ₃ iron oxide surfaces;
• The 4A to 4E represent atomic structures of iron oxide surfaces including an -OH termination;
• The 5 Figure 12 is a graph showing the surface energy of iron oxide surfaces as a function of increasing -OH termination;
• The 6A and 6B are graphs plotting the lattice parameter c and the calculated band gap of Fe ₂ O ₃ when different U-values are applied to Fe in density functional theory (DFT) calculations;
• The 7A to 7D show graphs of the density of states (DOS) of Fe ₂ O ₃ at different U-values; and
• The 8A and 8B show examples of powder X-ray diffraction (XRD) graphics for two different Fe ₂ O ₃ samples.

DETAILED DESCRIPTION

Embodiments of the present disclosure are described herein. It should be understood, however, that the disclosed embodiments are merely examples, and other embodiments may take various and alternative forms. The figures are not necessarily to scale; some features might be exaggerated or minimized to show details of specific components. Therefore, specific structural and functional details disclosed herein are not to be construed as limiting, but merely as a representative basis for instructing one skilled in the art to practice the present invention in various ways. As those of ordinary skill in the art will understand, various features illustrated and described with reference to any one of the figures may be combined with features illustrated in another or more others of the figures to provide embodiments that do not specifically illustrated or described. The combinations of features that are illustrated provide representative embodiments for typical applications. Various combinations and modifications of the features consistent with the teachings of this disclosure, however, may be desired for particular applications or implementations.

The description of a group or class of materials as suitable for a particular purpose in connection with one or more of the embodiments implies that mixtures of any two or more members of the group or class are suitable. Description of ingredients in chemical terms refers to the ingredients at the time of addition to any combination specified in the description and does not necessarily preclude chemical interactions among ingredients of the mixture after they have been mixed.

Unless expressly stated, all numerical quantities in this specification that indicate dimensions or material properties are understood to be modified by the word “about” in describing the broadest scope of the present disclosure.

The first definition of an acronym or other abbreviation applies to all subsequent uses of the same abbreviation herein and, when applied accordingly, applies to normal grammatical modifications of the abbreviation defined at the beginning. Unless expressly stated otherwise, the measured value of a property is determined by the same method as previously or later stated for the same property as a reference.

Particular reference is made to compositions, embodiments, and methods of embodiments known to the inventors. However, it should be understood that disclosed embodiments are merely examples of the present invention, which may be embodied in various and alternative forms. Therefore, the specific details disclosed herein are not to be construed as limiting, but merely as representative bases for teaching one skilled in the art how to use the present invention in various ways.

The term “substantially” or “about” may be used herein to describe disclosed or claimed embodiments. The term “substantially” or “about” may modify a value or a relative characteristic that is disclosed or claimed in the present disclosure. In such cases, “substantially” or “approximately” can mean that the value or the relative characteristic which it modifies is within ± 0%, 0.1%, 0.5%, 1%, 2 %, 3%, 4%, 5% or 10% of the value or the relative characteristic feature.

Corrosion is a natural process that converts a refined metal into a more chemically stable form, such as the oxide (s), hydroxide (s), sulfide (s) and / or other salts of the metal. The conversion represents a gradual destruction of the metal material caused by electrochemical oxidation of the metal upon reaction with an oxidizing agent such as oxygen or sulfates. Corrosion can result from exposure of the metal substrate to moisture in the air, a solution with a relatively low pH, various chemical substances such as acids, microbes, elevated temperatures, and / or other factors. Particularly in acidic environments, corrosion begins at the interface between the bulk of a metal material (e.g. steel) and a solution (e.g. ions dissolved in water or a surface layer of water which react to decompose the bulk material).

Due to the highly acidic operating environment, corrosion resistant metals, metal surfaces, treatments, and coatings are of particular use in proton exchange membrane fuel cells (PEMFC). PEMFCs typically operate under acidic conditions, with the pH usually between 2 and 4. Operating temperatures within a PEMFC stack range from approximately 60 to 85 ° C. These and other factors contribute to the highly corrosive operating environment of PEMFCs. For example, between startup and shutdown there are low voltages within the PEMFC stack, and fluorine ions are released during PEMFC operation as a result of the degradation of the Polymer membrane released. Furthermore, both H ₂ and O _{2 are present} at the anode during start-up and shutdown, which causes a high cathodic potential, which leads to cathodic corrosion. In light of such conditions, PEMFCs require durable components that are able to withstand the corrosive operating environment.

Earlier fuel cell systems often used graphite for the bipolar plate (BPP) of the PEMFC because graphite can achieve high chemical stability and conductivity within the PEMFC environment. However, graphite is both brittle and expensive. Stainless steel is now widely recognized as one of the best candidates for BPPs because of its excellent mechanical stability, electrical and thermal conductivities, and its relatively easy manufacture. Stainless steel is, of course, the generic name for a number of different steel compositions. Stainless steels typically include at least 10% chromium (Cr), which can form a stable chromium oxide surface layer that is known to prevent “staining” of the metal surface. SS304 and SS316 are two of the most common compositions of stainless steel. SS304 contains 18% Cr and 8% nickel (N ₁ ). SS316 contains 16% Cr, 10% Ni and 2% molybdenum (Mo). Stainless steel compositions can be varied depending upon the nature of the particular application. Such a variation leads to pronounced mechanical stability, corrosion resistance and magnetic properties. As is known, apart from iron (Fe), Cr, Ni and Mo, other elements in stainless steel include: carbon (-0.03%), manganese (1 ~ 2%), silicon (0.5 - 2%), nitrogen (0.01-0.1%), copper (0.5-2%) and cobalt (<0.5%).

Although this is less the case than pure Fe, stainless steel is still susceptible to corrosion. Corrosion of stainless steel occurs when the metal is exposed to and reacts to water / air and various contaminants on the surfaces of the metal. Exposure to Fe to water and oxygen leads to the formation of rust, which is typically characterized by the formation of red colored oxide. Rust includes the oxidized forms of Fe - ie, hydrated iron (III) oxides (especially Fe ₂ O ₃ · xH ₂ O) and iron (III) oxide hydroxides (especially FeO (OH) and Fe (OH) ₃ ). In acidic environments the formation of such iron oxide complexes can accelerate and some of these oxides can further dissolve in the solution.

When the iron-based oxide films are formed on stainless steel BPPs, the contact resistance and electrical conductivity of the PEMFC are greatly affected. Most oxides, for example, are insulators and therefore have a negative effect on the electrical conductivity of the bulk material. This is particularly problematic in the case of BPPs that are designed to be highly electronically conductive. The formation of insulating oxide film layers on BPPs reduces the transfer of electrons, which can result in decreased PEMFC power output. Furthermore, corrosion films can grow over time, which leads to greater contact resistance. Significantly, if the products of such corrosion can be ionized (e.g., Fe ²⁺ or Fe ³⁺ ), an acidic solution containing such ions can be transported to other fuel cell components. Within PEMFC stacks, Fe dissolution can poison the Pt catalyst, for example, which leads to reduced reaction rates of H ₂ and O ₂ adsorptions, H ₂ O formation and poor fuel cell efficiency.

Various types of coatings have been developed with numerous efforts to prevent or slow the corrosion of metals. Examples include applied coatings, such as paint, coating or electroplating, enamel, reactive coatings, including corrosion inhibitors, such as chromates, phosphates, conductive polymers, surfactant-like chemicals, which are designed to suppress electrochemical reactions between the environment and the metal substrate, anodized surfaces and biofilm coatings. In the case of BPPs used in PEMFC stacks, corrosion resistance can be imparted through the use of treatment techniques such as the introduction of conductive oxide and / or nitride coatings on the stainless steel. Despite such techniques, the corrosive formation of Fe ₂ O _{3 is} inevitable when Fe metal is exposed to water and oxygen, as is the case under the acidic operating conditions of the PEMFC environment.

As described in the present disclosure, some surface Fe ₂ O ₃ species in stainless steel - and in other metal compositions primarily composed of Fe - are more resistant to metal dissolution. Density functional theory (DFT) calculations based on the basic principles can be used to determine the relevant energetics of Fe ₂ O ₃ oxide surfaces. In accordance with the present disclosure, the formation of corrosion-resistant Fe ₂ O ₃ surfaces can be carefully controlled and transferred to stainless steel BPPs by using the narrow Precisely adjusts the range of Fe ₂ O ₃ surface energies that are sensitive to synthesis conditions and the local environment.

A non-limiting example of a proton exchange membrane fuel cell is shown in FIG 1 shown. A core component of the PEMFC 10 is a membrane electrode assembly (MEA) 12, which supports the electrochemical reaction within the stack. The MEA 12 includes sub-components such as electrodes, catalysts and proton exchange membranes. In addition to the MEA 12, the PEMFC 10 typically includes other components such as current collectors 14, gas diffusion layer (s) 16, cuffs 18, and at least one bipolar plate (BPP) 20.

The BPP 20 is implemented in a PEMFC stack for gas distribution, power generation, and separation of individual cells in the stack. The BPP 20 also provides additional functions such as removal of reaction products and water, as well as thermal management within the PEMFC 10. The BPP 20 is thus an essential part of the PEMFC 10. The BPP 20 is also both a relatively expensive component and a common reason for PEMFC system deterioration. For example, BPPs can make up about 60 to 80% of the stack weight of the PEMFC 10, about 50% of the stack volume, and about 25-45% of the stack cost. The BPP 20 poses another challenge to the material, since the BPP 20 must also be electrically conductive enough to enable electron transfer for the oxygen reduction reaction. Accordingly, the material of the BPP 20 should be both electrically conductive and chemically inert to reactions with ions present in the PEMFC 10 environment.

The metal surface of the BPP 20, which may include stainless steel, may include a coating such as a graphite-like coating or a protective oxide and / or nitride coating to increase the corrosion resistance of the BPP 20. The BPP 20 surface can thus include elements such as Fe, Cr, Ni, Mo, Mn, Si, P, C, S or a combination thereof. Alternative coatings include a Ti alloy, doped TiO _x , TiN, CrN, or ZrN. Again, even when such coatings are employed, in a harshly corrosive environment such as that of PEMFC 10, materials, structures, and techniques for increasing the corrosion resistance of BPP materials (e.g., stainless steel) are desirable. In accordance with embodiments set forth herein, materials, structures, and techniques for increasing the corrosion resistance of stainless steel BPPs are disclosed.

A non-limiting example of a bipolar plate (BPP) 20 of the PEMFC is shown in FIG 2 shown. The BPP 20 represents a non-limiting example of a substrate having a solid or mass region 22 and a surface region 24. The mass region 22 can be formed from a metal such as stainless steel. Alternatively, the mass region 22 can be formed from graphite, steel, aluminum, copper, an alloy of two or more metals, a combination thereof, or a composite material. The surface area 24 may include a corrosion resistant iron oxide film structure according to one or more of the embodiments. The entire area of the surface area 24 may include the iron oxide film structure. Alternatively, the surface area 24 may include one or more sub-areas that are free of the iron oxide film structure. In one embodiment, the entire surface area 24 includes the iron oxide film structure. The surface area 24 may further include protective coatings, such as binary oxide coatings, applied in the nanoscale to micron range (a few nm to 100 µm) on top of the corrosion resistant iron oxide film structure. Such binary oxide coating materials include, but are not limited to, MgO, Al ₂ O ₃ , TiO _2, and ZrO ₂ . These oxide coating materials can be undoped and / or partially doped with nitrogen, carbon or fluorine in order to further increase the resulting electrical conductivity. Still further, surface area 24 may include protective coatings formed from ternary (or higher) chemical compounds such as ABO _x , where A is Mg, Al, Ti or Zr, B is Zn, Sn, Cr or Mo and x is an integer ranging from 1 to 8. In accordance with some embodiments, the protective coating can include conductive nitrides and / or carbides.

The thickness of the surface area 24 and its associated corrosion-resistant iron oxide film layer can be adapted according to the specific application and can range from a few nm to about 1 μm. The iron oxide film layer itself can also range in thickness from about 1 nm to about 1 µm. Non-limiting examples of such thicknesses can be about 0.1 to 0.8 µm, 0.2 to 0.6 µm, or 0.3 to 0.5 µm. In one or more embodiment (s), the thickness of the corrosion-resistant iron oxide film can be between 1 nm and 0.5 μm. In other embodiments, the thickness of the corrosion-resistant iron oxide film can be between 150 nm and 0.3 µm.

Surfaces made of α-Fe ₂ O ₃ iron oxide can be characterized by their atomic morphological structures. Such surface facet structures can be defined by the following Miller indices: (001), (110), (100), (101) and (012). Each of these α-Fe ₂ O ₃ morphological surfaces is in the 3A to 3E shown. The 3A illustrates the atomic structure of (001) Fe ₂ O ₃ . The 3B Fig. 11 illustrates the atomic structure of (110) -Fe ₂ O ₃ . The 3C illustrates the atomic structure of (100) -Fe ₂ O ₃ . The 3D illustrates the atomic structure of (101) -Fe ₂ O ₃ . Finally illustrates the 3E the atomic structure of (012) -Fe ₂ O ₃ . The mass region of Fe ₂ O ₃ is made up of FeO ₆ octahedra, while the surface region is undercoordinated. As in the 3B and 3C For example, (110) and (012) iron oxide surfaces are both terminated with FeO ₅ polyhedra. Like in the 3D showed that (101) iron oxide has both FeO ₄ and FeOs surface units. (100) and (001) iron oxides are even more undercoordinated on their surface than (101) iron oxide. The 3C shows that (100) iron oxide is terminated with FeO ₄ , and the 3A shows that (001) iron oxide is terminated with FeO ₃ .

wobei E_0,Platte die gesamte interne DFT-Energie der spezifischen Fe₂O₃-Platte ist, wie in den 3A bis 3E gezeigt, E_0,Masse ist die interne DFT-Energie des Massen-Fe₂O₃ je Formeleinheit, n ist die Zahl der Formeleinheiten in der Plattenkonstruktion und A ist der Oberflächenbereich einer spezifischen Facette der Plattenkonstruktion. Die Wirkung der Wasser-Oxid-Grenzfläche soll ungefähr die gleiche für alle Eisenoxidoberflächen, die in den 3A bis 3E gezeigt sind, sein. Beispiele von berechneten DFT-Oberflächenenergien der verschiedenen in den 3A bis 3E dargestellten Fe₂O₃-Oberflächen sind in der nachstehenden Tabelle 1 gezeigt. Tabelle 1 Berechnete DFT-Oberflächenenergien von verschiedenen Fe₂O₃-Oberflächen α-Fe₂O₃-Oberflächenfacetten (001) (110) (100) (101) (012) Oberflächenenergie, γ (J/m²) 0,82 0,92 1,21 1,06 0,63 The density functional theory (DFT) surface energy of the iron oxide-Fe ₂ O ₃ surfaces that are present in the 3A to 3E can be calculated by applying the generalized gradient approximation (GGA) scheme. The 'Vienna Ab initio Simulation Package (VASP)' software can be used to perform such surface energy calculations. Surface energy, y, is the amount of energy required to cleave a bulk sample, creating two vacuum-exposed surfaces. A lower surface energy represents an energetically more favorable state and a higher surface energy represents an energetically less favorable state. The DFT surface energy Fe ₂ O ₃ (γ) can be calculated based on the following equation (1): $$\mathrm{\gamma \; =}\left({\mathrm{E.}}_{\mathrm{0,}Platte}=n\cdot {\mathrm{E.}}_{\mathrm{0,}\mathrm{M.}asse}\right)/\left(2\mathrm{A.}\right)$$

where E _{0, plate is} the total internal DFT energy of the specific Fe ₂ O ₃ plate, as in FIGS 3A to 3E shown, E _{0, mass} is the internal DFT energy of the mass Fe ₂ O ₃ per formula unit, n is the number of formula units in the plate construction and A is the surface area of a specific facet of the plate construction. The effect of the water-oxide interface is said to be roughly the same for all iron oxide surfaces in the 3A to 3E are shown. Examples of calculated DFT surface energies of the various in the 3A to 3E Fe ₂ O ₃ surfaces shown are shown in Table 1 below. Table 1 Calculated DFT surface energies of various Fe ₂ O ₃ surfaces α-Fe ₂ O ₃ surface facets (001) (110) (100) (101) (012) Surface energy, γ (J / m ² ) 0.82 0.92 1.21 1.06 0.63

As shown in Table 1, (012) -Fe ₂ O _{3 is} the energetically most favorable surface facet structure, followed by (001) - and (110) -Fe ₂ O ₃ . (101) - and (100) -Fe ₂ O ₃ show the highest DFT surface energies and are therefore less advantageous than the other iron oxide surface structures.

In Fe-based metals such as stainless steel, rust is generated according to the following reactions. Specifically, when exposed to water and air, Fe ₂ O ₃ forms Fe ₂ O ₃ xH ₂ O, as set out below: Fe _(s) → Fe ²⁺ _(aq.) + 2e ^- (A, oxidation half-reaction) O _{2 (g)} + 2H ₂ O ₍₁₎ + 4e ^- → 4OH ^- _(aq.) (B, reduction half-reaction) 2Fe _(s) + O _{2 (g)} + 2H ₂ O ₍₁₎ → 2Fe ²⁺ _(aq.) + 4OH ^- _(aq.) (2A + B) Fe ²⁺ _(aq.) + 2OH ^- _(aq.) → Fe (OH) _{2 (s)} (intermediate state) 4Fe (OH) _{2 (s)} + O _{2 (g)} + xH ₂ O _(I) → 2Fe ₂ O ₃ (x + 4) H ₂ O _(s) (further reaction with H2O and O2)

In view of the above reaction processes in the formation of rust in Fe-based metals such as stainless steel, the atomic structure of iron oxide-Fe ₂ O ₃ surfaces can be characterized by -OH termination. Non-limiting examples of the atomic structures of iron oxide surfaces that include an -OH termination are given in US Pat 4A to 4E shown. The 4A Figure ₃ illustrates the atomic structure of (001) -Fe ₂ O ₃ including an -OH termination 4B Figure ₃ illustrates the atomic structure of (110) -Fe ₂ O ₃ including an -OH termination 4C Figure ₃ illustrates the atomic structure of (100) -Fe ₂ O ₃ including an -OH termination 4D Figure ₃ illustrates the atomic structure of (101) -Fe ₂ O ₃ including an -OH termination 4E represents the atomic structure of (012) -Fe ₂ O ₃ , which includes an -OH termination.

A graph showing the surface energy of iron oxide surfaces as a function of increasing -OH termination is in FIG 5 shown. The graph shows changes in particle morphologies and affected surface energies when an -OH termination occurs in the 3A to 3E Fe ₂ O ₃ surface facets disclosed is introduced and increased therein. The calculations that make up the graph of 5 were performed as described in connection with Table 1 above. Accordingly, the density functional theory (DFT) surface energies of the iron oxide-Fe ₂ O ₃ surfaces have been calculated by applying the generalized gradient approximation (GGA) scheme. The particle shape can be derived from the calculated surface energies with the help of the well-known Wulff construction. In line with experimental results, the demonstrated 5 that both cubic and hexagonal forms of Fe ₂ O ₃ can be observed. The 5 also shows that under dry conditions Fe ₂ O _{3 has} a cubic (or pseudo-cubic) shape and is dominated by (110) and (012) -Fe ₂ O ₃ surface facets. Increased -OH terminations can result from humid conditions - where H ₂ O is present - and / or acidic conditions - where available protons (H ⁺ ) have reacted with surface oxygen atoms in Fe ₂ O ₃ . Fe ₂ O ₃ particles of different sizes and morphology can be obtained by controlling the nucleation or nucleation time. Like in the 5 shown, the particle shape of -OH-terminated Fe ₂ O ₃ systems is hexagonal, with the (001) surface facet dominating. This further explains why the most experimentally studied Fe ₂ O ₃ surface is (001) -Fe ₂ O ₃ .

How with the 3-5 and the related disclosure demonstrated, DFT calculations according to the basic principles can exactly reproduce the Fe ₂ O ₃ morphology observed in the experiment within a generalized gradient approximation (GGA). However, more advanced calculation methods can take into account the error of overdelocalization of electrons that are present in the simple GGA approach.

Graphs in which the lattice parameter c and the calculated band gap of an Fe ₂ O ₃ mass structure are plotted, with different U-values applied to Fe in the DFT calculations, are in FIG 6A and 6B shown. For example, as shown in the figures, when U _Fe = 8 eV is applied, both the lattice parameter c ( 6A , -13.7 Å) and the experimental band gap ( 6B , ~ 2.0 eV) can be recorded. As the value for U _{Fe increases} in the DFT calculations, both the lattice parameter and the band gap approach the experimental values. Typically, U _Fe = 4 eV is conventionally used for the DFT calculation, this being adapted to the experimental formation energy of iron oxides. How with the 6A and 6B however, as demonstrated, the inclusion of U _Fe = 8 eV can more accurately reflect the actual Fe ₂ O ₃ system, considering the close agreement between the experimental and calculated lattice parameter and band gap when U _Fe = 8 eV.

Graphs of the density of states (DOS) of Fe ₂ O ₃ at different U-values are in the 7A to 7D shown. As shown, the bandgap opens as the U-values increase. The band gap (E _g ) is expressed as the distance between the Fermi level, E _F (x = 0), and the conduction band (where x is a positive number in the graphs of FIG 7th is defined. The 6B and 7A to 7D show, for example, that pure GGA and U = 3 have no band gap, while higher U values, such as U _Fe = 6 and 8, show that the distance (near x = 0) between the occupied and unoccupied states (i.e. the band gap ) has increased.

In view of the parameters discussed above for the exact representation of the Fe ₂ O ₃ system - both structurally (particle shapes) and electronically (density of states) - the energy required to remove an Fe atom in each system can be estimated from (101) -, (001) -, (110) -, (012) - and (100) -Fe ₂ O ₃ can be determined by calculation. Table 2 below sets out the lowest formation of Fe vacancies in the surface of the various Fe ₂ O ₃ surfaces described above. As shown in Table 2, it would be hardest to remove Fe from the (110), (012) and (100) Fe ₂ O ₃ surfaces, and comparatively easier to remove Fe from (101) and (001) -Fe ₂ O ₃ surfaces to remove. Table 2 Calculated formation of Fe vacancies in DFT surfaces of various Fe ₂ O ₃ surfaces α-Fe ₂ O ₃ surface facet ΔE _{Vak, Fe, surface} Surface termination (101) 1.457 FeO ₄ and FeO ₅ (001) 2.728 FeO ₃ (110) 3.057 FeO ₅ (012) 4,331 FeO ₅ (100) 5,987 FeO ₄

wobei E_0,w/FeVakanz die interne DFT-Energie des Platten-Fe₂O₃ ist, wobei eines der Oberflächen-Fe-Atome entfernt wurde, E_{0,ursprünglich} die interne DFT-Energie der ursprünglichen Platte ist und µ_Fe das chemische Potential von Fe ist, bestimmt anhand von bcc- bzw. kubisch raumzentriertem Fe-Massenmetall. Höhere Energien der Bildung von DFT-Vakanzen zeigen eine erhöhte Schwierigkeit in Zusammenhang mit dem Entfernen eines Fe-Atoms aus dem System an.The energy associated with the formation of Fe vacancies in DFT surfaces was determined based on the following equation (2): $$\Delta {\mathrm{E.}}_{\text{Vac}\text{, Fe}\text{,Surface}}={\mathrm{E.}}_{\mathrm{0,}w/\mathrm{F.}eVakanz}-\left({\mathrm{E.}}_{\mathrm{0,}ursprünGlicH}+{\mu }_{\text{Fe}}\right)$$

where E _{0, w / FeVakanz is} the internal DFT energy of the plate Fe ₂ O ₃ , where one of the surface Fe atoms has been removed, E _{0, originally} the internal DFT energy of the original plate and µ _Fe the chemical The potential of Fe is determined on the basis of bcc or body-centered cubic Fe bulk metal. Higher energies of DFT vacancy formation indicate an increased difficulty associated with removing an Fe atom from the system.

According to the data recorded in Table 2, if the Fe ₂ O ₃ film having (110), (012) and (100) surface facets can be grown, the Fe dissolution can be compared with the Fe ₂ O ₃ film, which is dominated by (101) and (001) surface facets, will be more difficult. As in the 5 As shown, cubic-shaped Fe ₂ O _{3 is} largely dominated by (110) and (012) surfaces, while the hexagonal-shaped Fe ₂ O ₃ includes primarily (001) and (110) surfaces. The preparation and selection of Fe ₂ O ₃ surfaces that include increased resistance to Fe dissolution can be used to properly identify and apply corrosion resistant oxide films for use within the bipolar plates (BPPs) of PEMFC stacks. And while stainless steel corrodes to Fe ₂ O ₃ , the use of highly resistant Fe ₂ O ₃ surfaces can prevent (or at least slow down) further Fe ion dissolution in the acidic operating environment of PEMFCs. Stainless steel BPPs with protective Fe ₂ O ₃ surface layers comprising dissolution-resistant (110), (012) and (100) facets and ranging in thickness from a few nm to about 1 μm can minimize and / or dissolve reactions that lead to the formation of species - such as radicals - that can cause degradation of the polymer membrane and catalysis in PEMFCs. Poisoning of a Pt catalyst initiated by Fe dissolution can be suppressed, for example, by using more stable Fe ₂ O ₃ surfaces within the BPP, thereby increasing the potential life of PEMFCs.

In one or more of the embodiments, stainless steel with a corrosion-resistant surface oxide layer comprising (110), (012) and / or (100) -Fe ₂ O ₃ surface facets is used for a PEMFC-BPP. In other embodiments, stainless steel with a corrosion resistant surface oxide layer comprising (001) -Fe ₂ O ₃ surface facets is used for a PEMFC-BPP. Other favorable surface facets can be a family of lattice planes of (110), (012), (100) and / or (001), such as (006), (113), (024), (116), (122), (213 ), (300), etc. As described above, the thickness of the corrosion-resistant oxide layer can be adjusted according to the requirements of a specific application and can range from about 1 nm to about 1 μm. Non-limiting examples of such thicknesses can be about 0.1 to 0.8 µm, 0.2 to 0.6 µm, or 0.3 to 0.5 µm. In certain embodiments, the thickness can be between 1 nm and 0.5 µm. In other embodiments, the thickness of the corrosion-resistant iron oxide film can be between 150 nm and 0.3 µm.

According to certain embodiments, a stainless steel substrate is provided with a corrosion resistant surface oxide layer defined by a surface morphology that includes a first surface facet group that includes (110), (012) and (100) Fe ₂ O ₃ surfaces, and a second Surface facet group including (001) and (101) -Fe ₂ O ₃ surfaces are disclosed. In some embodiments, the first surface facet group can exclusively include (110), (012), or (100) -Fe ₂ O ₃ surfaces. In other embodiments, the first surface facet group can include two or more of the (110), (012), and (100) -Fe ₂ O ₃ surface structures. Similarly, in some embodiments, the second surface facet group may exclusively include (001) or (101) Fe ₂ O ₃ surfaces. In other embodiments, the second surface facet group can include both (001) and (101) -Fe ₂ O ₃ surface structures. In certain embodiments, the Fe ₂ O ₃ oxide layer comprises predominantly Fe ₂ O ₃ surface structures of the first surface facet group. In some embodiments, the stainless steel substrate can include a corrosion resistant surface oxide layer characterized by a surface morphology comprising 70% to 90% of the first surface facet group and 10% to 30% of the second surface facet group. Thus, according to certain embodiments, the ratio of the first surface facet group to the second surface facet group can be in the range from 9: 1 to 7: 3. In other embodiments, the stainless steel substrate may include a corrosion resistant surface oxide layer characterized by a surface morphology that comprises greater than 90% of the first group of surface facets. In at least one further embodiment, the corrosion-resistant surface oxide layer can be characterized by a surface morphology which comprises more than 95% of the first surface facet group. Other embodiments may include a corrosion-resistant surface oxide layer characterized by a surface morphology that includes 100% of the first surface facet group. Some embodiments may include an amorphous FeO _x surface structure, where x ranges from 1 to 2. Other metal contaminants can also be included. Such metals include, but are not limited to, Cr, Ni, Co, Mn and Si.

The presence of different Fe ₂ O ₃ surface facets can be verified using powder X-ray diffraction (XRD) or a high-resolution transmission electron microscope (HR-TEM). For example, the (012) peak for Fe ₂ O _{3 is located} between 24 to 26 ° 2θ, (110) is located between 35 to 38 ° 2θ and (300) is located between 62 to 65 ° 2θ when using XRD a Cu-Kα source (λ = 1.54 Å) is measured. The relative ratio of different planes can also be quantified using the relative XRD height and / or the full width at half maximum. Fe ₂ O ₃ with more (110) - and (012) - [fractions] can be identified by HR-TEM with a d-spacing of -0.25 nm, whereby such structures can appear as a cubic or pseudocubic shape. Of course, after more (001) -Fe ₂ O _{3 has} formed, Fe ₂ O _{3 can appear} as a hexagonal shape.

Non-limiting examples of powder X-ray diffraction (XRD) charts for two different Fe ₂ O ₃ samples are provided in US Pat 8A and 8B shown. The 8A and 8B show two examples of Fe ₂ O ₃ with different XRD peak intensities. As described above, corrosion-resistant oxide films can primarily include (110), (012) and / or (100) -Fe ₂ O ₃ surface facets and the lattice planes of their families (e.g. (024), (300), etc.) . The 8A and 8B show that the relative ratio of different planes in XRD patterns can differ, which can be quantified using either the XRD height and / or the full width at half maximum. In this case the 2 theta values in the x-axis are given by using Cu as the X-ray source, in particular the Kα radiation from a 1.54 Å source.

In one or more of the embodiments, stainless steel BPPs utilizing the disclosed corrosion resistant oxide surface layers comprise at least 10 to 20% chromium (Cr) and 5 to 10% nickel (Ni). Other elements in the stainless steel can be molybdenum (~ 1 to 2%), carbon (-0.03%), manganese (1 ~ 2%), silicon (0.5-2%), nitrogen (0.01-0 , 1%), copper (0.5-2%) and cobalt (<0.5%) include, but are not limited to, the remainder being iron (Fe). A stable Cr oxide film (in addition to the special Fe ₂ O ₃ oxide films described herein) can also be present on the BPP surface in order to slow down the corrosion of the BPP material. In some embodiments, other crystalline and / or amorphous metal oxides, including but not limited to NiO, MoO ₂ , MoO ₃ , MnO, Mn ₂ O ₃ , MnO ₂ , SiO ₂ , CuO, Co ₃ O ₄ , etc., can be included , can also be used to suppress the corrosion of the BPP material.

Because of their composition, structure, and morphology, BPPs constructed from the Fe ₂ O ₃ surfaces and coatings disclosed in accordance with certain embodiments can include a number of desirable properties. BPPs constructed from the Fe ₂ O ₃ surfaces and coatings disclosed herein can, for example, have corrosion resistances of less than about 1 µA cm ^-2 at 80 ° C, at pH = 2 to 3 in the presence of about 0.1 Show ppm HF in the solution. In other embodiments, the BPP oxide coatings can have a corrosion current of at least less than about Reach 0.5 to 10, 1 to 8 or 1.5 to 5 µA cm ^-2 under the same operating conditions. The electrical conductivities of the BPP oxide coatings can be greater than 100 S cm ^-1 , and the thickness of the coating layer can be optimized in order to achieve the target conductivity. In certain embodiments, the electrical conductivities of the BPP oxide coatings are between 100 and 150, 110 and 140 or 120 and 130 S cm ^-1 . In other embodiments, the electrical conductivities of the BPP coatings are between 0.1 and 100, 1 and 80 or 10 and 50 S cm ^-1 . The contact resistance of the interfaces between the stainless steel substrate and the given BPP coating can be less than about 0.01 ohm cm ² . In certain embodiments, the contact resistance of the interfaces is between 0.001 and 0.2, 0.005 and 0.1, or 0.01 and 0.05 ohm cm ² .

A number of methods for growing the corrosion resistant oxide film layers are also disclosed herein. In at least one embodiment, the method includes growing the Fe ₂ O ₃ oxide film on the stainless steel using a solution-based method. Hydrolysis can be carried out at 80 to 100 ° C in a water bath with different aging times of 1, 2, 3, 4, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55 or 60 minutes will. In one or more of the embodiments, hydrolysis is carried out at 25 to 100 ° C. in a water bath with an aging time of between 1 minute and 120 minutes. The reaction time can also vary from 2 to 96 hours and, according to some embodiments, can be between 24 and 72 hours. The presence of a containing precursor such as FeCl ₃ with an acid such as HCl, HNO ₃ , H ₂ SO ₄ can additionally help control the nucleation of various surface facet formations. The solution-based process can further include acid and / or base treatment with another type of oxidizing and / or reducing chemical agent.

An alternative method of growing the Fe ₂ O ₃ oxide film (s) disclosed herein includes electrochemical methods. According to such a method, stainless steel can be polished and cleaned with an organic solvent such as ethanol and then oxidized electrochemically. The working electrode is typically stainless steel, and the counter and reference electrodes can vary depending on the voltage windows. Typically, Pt foil and / or Ag / AgCl (with saturated KCl) can be used as counter and reference electrodes. The immersed electrolytic solution can be an acid with a varied concentration (e.g. 0.01 to 1 M sulfuric acid), the exact pH (which ranges from pH 1 to 13) being adjusted or neutralized as required . The electrochemical process can also include, but is not limited to, the use of acidic and / or basic solutions such as HCl, H ₂ SO ₄ , HClO ₄ , NaOH and KOH.

Yet another method of growing the Fe ₂ O ₃ oxide film (s) disclosed herein includes heat treatment. Stainless steel can be heat treated in a box furnace at temperatures varying from 150 to 900 ° C in the presence of a mild oxidizing agent such as air or oxygen. The heat treatment process can further include adjusting the heating and / or cooling rate to 1 to 10 degrees per minute. The cooling process can be accomplished by a natural cooling or a rapid quenching step.

While the BPP of a PEMFC has been described as a suitable application for the corrosion-resistant iron oxides set out above, the oxide layers disclosed may be suitable for other applications as well. For example, the disclosed corrosion resistant oxide films can be used as part of a surface area in other industrial applications that require a chemically inert, conductive material, such as batteries, photovoltaics, consumer electronics, and / or where otherwise a conductive and inert oxide is advantageous would. Furthermore, a surface area of an applicable device may include a relatively thin, corrosion-resistant oxide film such that the film is transparent. The material can thus act as a transparent conductive oxide film that can be used, for example, in photovoltaics.

While exemplary embodiments have been described above, it is not intended that these embodiments describe all possible forms of the invention. Rather, the wording used in the specification is descriptive and not restrictive, and it is understood that various changes can be made without departing from the spirit and scope of the invention. Furthermore, the features of different implementations of the embodiments can be combined to form further embodiments of the invention.

Claims (20)

A corrosion resistant substrate comprising: a mass range; and a surface region including an Fe ₂ O ₃ oxide layer having (110), (012), or (100) Fe ₂ O ₃ surface facets configured to impart corrosion resistance properties to the substrate; wherein the Fe ₂ O ₃ oxide layer is between 0.001 and 0.5 µm thick and the (110), (012) and (100) -Fe ₂ O ₃ surface facets cover more than 50% of the surface area of the substrate. Substrate of Claim 1 wherein the surface area includes a protective coating comprising MgO, Al ₂ O ₃ , TiO ₂ or ZrO ₂ . Substrate of Claim 1 wherein the surface area further includes a protective coating having a structure described by: ABO _x , where A is Mg, Al, Ti or Zr, B is Zn, Sn, Cr or Mo and x is an integer ranging from 1 to 8. Substrate of Claim 1 wherein the mass range is stainless steel comprising between 10 and 20% chromium and 5 to 10% nickel. Substrate of Claim 1 , the electrical conductivity of which is greater than about 100 S cm ^-1 . Substrate of Claim 1 wherein the surface area includes a corrosion resistance of less than 1 µA cm ^-2 at 80 ° C, a pH between 2 and 3, and in the presence of about 0.1 ppm HF. Substrate of Claim 1 , the Fe ₂ O ₃ oxide layer being between 0.15 and 0.3 µm thick. Substrate of Claim 1 wherein the Fe ₂ O ₃ oxide layer includes a first surface facet group, the (110), (012) or (100) -Fe ₂ O ₃ surface facets, and a second surface facet group, the (001) or (101) -Fe ₂ O ₃ surface facets. Substrate of Claim 8 wherein the Fe ₂ O ₃ oxide layer comprises between 70% and 90% of the first surface facet group and between 10% and 30% of the second surface facet group. A bipolar plate for a proton exchange membrane fuel cell comprising: a metal substrate having a mass area and a surface area including an Fe ₂ O ₃ oxide layer with (110), (012), or (100) Fe ₂ O ₃ surface facets, configured to impart corrosion resistance properties to the substrate; the Fe ₂ O ₃ oxide layer being between 0.001 and 0.5 µm thick and the corrosion resistance of the surface area being less than 1 µA cm ^-2 at 80 ° C, a pH range between 2 and 3 and in the presence of about 0 , 1 ppm HF. Bipolar plate of Claim 10 , wherein the electrical conductivity of the metal substrate is greater than about 100 S cm ^-1 . Bipolar plate of Claim 10 wherein the mass range is stainless steel which comprises between 10 and 20% chromium and 5 to 10% nickel. Bipolar plate of Claim 10 wherein the surface area further includes a protective coating comprising MgO, Al ₂ O ₃ , TiO _2, or ZrO ₂ . Bipolar plate of Claim 10 wherein the surface area further includes a protective coating having a structure described by: ABO _x , where A is Mg, Al, Ti or Zr, B is Zn, Sn, Cr or Mo and x is an integer ranging from 1 to 8. Bipolar plate of Claim 10 , the Fe ₂ O ₃ oxide layer being between 0.15 and 0.3 µm thick. Bipolar plate of Claim 10 wherein the Fe ₂ O ₃ oxide layer includes a first surface facet group, the (110), (012) or (100) -Fe ₂ O ₃ surface facets, and a second surface facet group, the (001) or (101) -Fe ₂ O ₃ surface facets. Bipolar plate of Claim 16 wherein the Fe ₂ O ₃ oxide layer comprises between 70% and 90% of the first surface facet group and between 10% and 30% of the second surface facet group. A method of making a corrosion-resistant stainless steel substrate having a bulk area and a surface area including an Fe ₂ O ₃ oxide layer having (110), (012), or (100) Fe ₂ O ₃ surface facets, the method Comprises: cleaning a stainless steel substrate with an organic solvent; and electrochemically oxidizing the stainless steel substrate to form a corrosion resistant surface area comprising an Fe ₂ O ₃ oxide layer between 0.001 and 0.5 µm thick with (110), (012) or (100) -Fe ₂ O ₃ -Includes surface facets covering greater than 50% of the surface area of the substrate. Procedure according to Claim 18 , further comprising depositing a protective coating comprising MgO, Al ₂ O ₃ , TiO ₂ or ZrO ₂ on the surface area of the substrate. Procedure according to Claim 18 , further comprising depositing a protective coating having a structure described by: ABO _x , where A is Mg, Al, Ti or Zr, B is Zn, Sn, Cr or Mo and x is an integer ranging from 1 to 8.

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